The present invention relates to internally threaded metallic inserts for plastics, manufactured from a raw metal rod and designed to provide stronger threads in a weaker host material.
The common method of providing thread load performance to accepted engineering standards in plastic sections is through the use of internally threaded metallic inserts. These inserts are usually manufactured from non-ferrous materials such as brass and aluminum, but may be manufactured from ferrous materials such as steel and stainless steel. They can be molded into plastic sections during their creation (known as molded in), or they can be installed into a preexisting cavity in an already molded part using axial pressure (known as post molding). Post molding techniques can include preheating and ultrasonic to sufficiently heat the insert enough to transfer that energy to soften the plastic material allowing it to flow fully in and around the external profiles of the insert as it is inserted into the hole. This creates maximum potential for torque and pull out resistance up to the design limitations of the insert.
Inserts are most commonly manufactured by the metal turning process using a 12 to 16 foot long bar. The machinery is generally an automatic screw machine, a CNC machine, or a rotary transfer machine. Some inserts are manufactured using turning equipment employing coiled material.
Some inserts are created via a zinc die casting technology which can cast an identical shape to the inserts being applied for protection in this patent application.
Standard to the industry, the performance of an insert is created by features machined onto the outside profile of a round, hexagon or other shaped bar or coil of raw material. These features have two distinctly different purposes but they work in conjunction with each other. One purpose is resistance to pull out so the insert stays in the hole. Those features are machined around the circumference and perpendicular to the thread axis and are referred to as ribs, undercuts, grooves and barbs. They grip by allowing the host material to flow in and around their shape during installation to provide the high resistance to pullout due to their configurations. The second is resistance to torque so the insert does not spin in the host material as the mating fastener is either tightened or loosened. This feature is most commonly rolled onto the insert as knurling, or points around its periphery, or machined on from the end as a broaching operation. Knurling and broaching create this sharp edged series of points around the periphery of the part which dig into the host and prevent the insert from turning as the torque is applied through the mating fastener.
Other methods sometimes used to create torque resistance can be the shape of standard hexagon or square bar but these methods have the limitation of being much larger in diameter as the circle around the points of the hex or square requires a much wider boss to contain them than does a round bar with knurls that is more compact and narrow by comparison.
The most common forms of knurling are straight, helical (angular), or male or a female diamond pattern. Some manufacturers produce two opposing knurling patterns on the same part in an effort to resist torque during the assembly and disassembly process. To achieve the correct knurl shape and specification, knurling requires a precision diameter on which to roll or broach. Usually a precision knurl requires a forming operation to remove the bulk of material down to a smaller diameter, then a light precision shaving operation to bring the formed diameter to the pre-knurled design diameter, and then the rolling operation itself, forming the knurl into the metal surface to achieve an exact tooth count around the part and to create faithfully its cross sectional shape. These three processes are time consuming during production and are sequentially interdependent. If one of the three operations is out of specification, the subsequent operations and the final resulting knurl pattern are at risk of not meeting the design criteria. This can result in poor insert performance due to mis-tracked or double tracked knurls, broken knurls, slivers, poor displacement, and out of spec knurl diameter on the part. In total, these three processes can be the most difficult metal cutting operations to control consistently in high production high speed automatic screw machines yet they create one of the most important characteristics of the insert's design.